\ 226 ALASKAN GLACIER STUDIES
I oped in western North America, and doubtless in all mountain areas where the conditions
[ resemble those of Alaska. Any student of the phenomena to which these ancient ex-
panded glaciers gave rise receives aid from the results of the study of the living Alaskan representatives of the type.
Deposits of Piedmont Glaciers. The extensive crescentic moraines of the Alpine piedmont glaciers, the extensive development of associated outwash gravel deposits, and the depressions within the crescentic moraines, often occupied by lakes, are so like the Alaskan conditions as to make it certain that their interpretation will be aided by a knowledge of the Alaskan phenomena. This fact was vividly brought to the attention of the senior author in the eastern Alps when, in company with Professor Hans Crammer, he looked out over the site of the great piedmont bulb of the ancient Salzburg Glacier; and later when crossing the moraines of similar bulbs near Munich. In the one case we have the process in operation before our eyes; in the other the finished product; but as Penck and Brlickner, Crammer, and others have observed, the nature of the phenomenon is so clear in the finished product that one cannot misinterpret it. In the details, however, light must be thrown on the interpretation of these records of ancient piedmont bulbs from the facts of observations obtained from a study of the living instances.
Glacial Erosion. From Russell's * and Gilbert's 2 studies, and later from our own8 the Alaskan field has yielded facts of importance on the subject of glacial erosion, applicable to other regions of glaciation. Vast erosion has been performed, and many forms resulting from glacial sculpturing have been observed. This important subject is not specifically treated in this book, however, except incidentally in the description of certain Yakutat Bay glaciers and at some length for glacial erosion below sea level as revealed by new soundings in Yakutat Bay in 1910, discussed in an earlier part of this chapter, and in Part HE, dealing with Prince William Sound and Copper River.
The features of glacial erosion are so clearly and intensively developed in Yakutat Bay and other Alaskan fiords that applications, comparisons, contrasts, and interpretations in other regions are made easier by reason of the perfection and recency of development of the forms of glacial erosion in the Alaskan region.
Ineffectiveness of Weak Glaciers. In the Yakutat Bay region we have had three stages of glaciers (1) the present, dwindling glaciers, (2) former, greatly expanded ice streams, (S) an intermediate stage of rather weak expansion. The three stages clearly show differences in extent of glacial erosion. Profound work was performed by the greatly expanded glaciers, deepening valleys one or two thousand feet by rock excavation; the less powerful, briefer period of advance failed to remove even the unconsolidated gravels that the glaciers overrode. This aids in understanding the ineffectiveness of small dwindling glaciers of the present day. As Andrews has clearly shown we must distinguish between the ice flood and the present day glaciers, as we must between the stream flood and the low water stage of erosion. The phenomena of glacial erosion in Yakutat Bay throw jight on these differences. In the minor erosion of the gravels, by overriding, we have
i Russell, I. C., Nat. Geog. Mag., Vol. Ill, 1891, pp. 100, 191; Amer. Journ. Sci., 3rd Series, Vol. 48, 1892, p. 173; ISth Ann. Rept., U. S. Geol. Survey, Part II, 1892, pp. 86, 90.
i Gilbert, G. K, Earriman Alaska Expedition, Vol. HI, 1904, pp. 57, 118, 170.
• Tarr, R. S. and Martin, Lawrence, Bull. Amer. Geog. Soc., Vol. XXXVm, 1906, pp. 158-160; Tarr, R. S., Popular Science Monthly, Vol. 70, 1907, pp. 99-119; Scottish Geog. Mag., Vol. 24, 1908, pp. 575^587; Professional Paper 64, TJ. S. Geol. Survey, 1909, pp. 107-119.